The first was a great film during the time it got released. It introduced a lot of great film techniques and technology that would be used and emulated on other films.

Feeling the need to cash out on the momentum of the first film, they decided to make it a trilogy and rush the movies to the theaters as quick as possible. Needless to say, the 2nd and 3rd ended up being some overblown crap sci-fi flicks that somehow tarnished the reputation of the first film. If you truly enjoyed the first film, you'd definitely hate the sequels.